Convert |CXFA_FFDocView::m_IndexChangedSubforms| to a deque.

Comments

Convert |CXFA_FFDocView::m_IndexChangedSubforms| to a deque.
Pop off items one at a time to safely iterate through it. Add a set to
keep track of seen notes to prevent infinite loops.
BUG=chromium:932900
Change-Id: I74eb29262e8ba29097638f7103ef427799a06fc5
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/52971
Commit-Queue: Lei Zhang <thestig@chromium.org>
Reviewed-by: Tom Sepez <tsepez@chromium.org>